There are several systems, in use, where a terminal can receive signals from orbiting navigational satellites whereby the position of the terminal, on the surface of the earth, can be measured with accuracy extending down to a few meters. One such system is the NAVSTAR.TM. GPS (Global Positioning by Satellite) system. A terminal will receive signals from any navigational satellites which it can hear. The navigational satellites broadcast to all terminals in their vicinity. Messages pass only from the navigational satellite to the terminal. The satellite transmits its identity, the exact time at which the broadcast was sent, and copious details concerning the orbit of the navigational satellite. The terminal notes the time that the data was received from the navigational satellite and stores the identity and details of the orbit. The terminal resolves its own clock inaccuracies by processing data from additional satellites. By calculating the precise location of the navigational satellite, from the orbit data, at the instant when the broadcast was sent, and by comparing this with the time the broadcast was received, to establish the delay between when the broadcast was sent and when it was received, the terminal is able to calculate the propagation delay between the navigational satellite and itself. The propagation delay represents a measured distance between the navigational satellite and the terminal at the instant the broadcast was made. By listening to many different navigational satellites, the terminal is able to triangulate between them to provide a positional fix.
Effective though the GPS system undoubtedly is, it contains several disadvantages. Firstly, in order to avoid signal collision, the navigational satellites broadcast on different channels, separated by appropriately orthogonal spreading sequences. In order, when first switched on, for the terminal, not having any idea of its location or the present position of any navigational satellite, to establish its position, it is necessary for the terminal to listen for all navigational satellites on all known navigational satellite channels. Given that the GPS system has many satellites, and that only a handful of the navigational satellites are required for a positional fix, this represents a gross inefficiency and can require a "cold" terminal to wait for up to thirty minutes for a sufficient number of satellite channels to be scanned and for receiver clock inaccuracies to be resolved. The present invention seeks to provide a system and method, and a terminal for use in the system, whereby the "from cold" start-up time of a terminal can be reduced from a potentially long period to just a short period.
Because of its military origins, it is preferred that a GPS terminal should not transmit for fear of revealing the position of the terminal to an enemy. While the present invention seeks to incorporate the terminal within a satellite communications system, it nonetheless also seeks to provide a means whereby the start-up time of a cold terminal can be greatly reduced without any need for the terminal, in the satellite communications system, being required to transmit or being required to run orbit simulations for the navigational satellites.
The navigational satellites can move with considerable speed. If the navigational satellite is approaching a terminal, this results in a Doppler effect increase in the frequency of transmission of a broadcast signal. Likewise, if a navigational satellite is receding from a terminal, this results in a Doppler effect reduction on the frequency of a broadcast signal. The terminal cannot be certain exactly on what frequency the broadcast from the navigational satellite might be. The present invention seeks, by including the terminal within a communications satellite system, to remove this uncertainty so that acquisition of navigational satellite signals can rapidly be achieved, even without the necessity for the terminal to transmit.
The GPS terminal, in common with all terminals for navigational satellite position fixing systems, is a powerful data processor. It has to assemble the rather large amount of orbital data for each navigational satellite whose broadcasts it can receive. Thereafter, the terminal is required to process all of the data from many navigational satellites in order to calculate its position. While this may not be a problem for military systems where cost and size usually are not a problem, in the civilian market cost and size can be a paramount factor. This has the result that the terminal is often equipped only with a small microprocessor which, although quite capable of performing the necessary calculations and functions, takes a great deal of time to do so. The net result is a handheld terminal which, from cold, can take a very long time to present its user with a result. By incorporating the terminal within a communications network, the present invention seeks to relieve the terminal of this burden so that very simple and economic terminals can be employed.
Even a "hot" terminal can have a problem when it requires a position update. The terminal user may be in a hurry. The terminal user may not require a precise position fix. Nonetheless, this is all he or she can get. The terminal user has to wait until the terminal, even despite having a previous fix on its position, goes through all or part of the "from cold" routine. The present invention seeks to provide, by incorporating the terminal in a satellite communications network, a method and system whereby the terminal user can obtain a very rapid update or confirmation of his or her position, even faster that the fastest that can be obtained using the navigational satellites.
The terminal may find itself in a position of poor reception, such as when its radio paths are blocked by buildings, geographical features, and the like. Under these circumstances, it may not be possible for signals from a sufficient number of navigational satellites to be received for a position fix. The present invention seeks to provide means whereby the position of the terminal can be found provided the terminal has a view of, as a minimum, at least one navigational satellite and one communications satellite.